This invention relates to a novel process for producing cyanuric acid by pyrolytic conversion of a nitrogenous material such as urea.
It is known that cyanuric acid can be produced by the pyrolysis of urea. This reaction may be expressed by the equation: ##STR1## The resulting product, cyanuric acid, which has the emperical formula C.sub.3 H.sub.3 O.sub.3 N.sub.3, is generally represented structurally either as: ##STR2## or ##STR3## The pyrolysis can be carried out either in a dry state, that is, in the absence of a solvent, such as described in U.S. Pat. No. 2,943,088, issued to R. H. Westfall on June 28, 1960, or in the presence of various inert organic solvents, such as described in the U.S. Pat. Nos. 2,952,679, issued to Perret on Sept. 13, 1960 (dipropylene glycol); 3,065,223, issued to Hopkins et al on Nov. 20, 1962 (alkyl sulfones); 3,117,968, issued to Merkel et al on Jan. 14, 1974 (diphenyl); 3,164,591, issued to Walles et al on Jan. 5, 1965 (oxyzolidinones); 3,563,987, issued to Berkowitz on Feb. 16, 1971 (sulfolane); 3,635,968, issued to Goelz et al on Jan. 18, 1972 (N-cyclohexyl pyrrolidone); or 3,810,891, issued to Lee on May 14, 1974 (glycol ethers).
Unfortunately, the pyrolysis and condensation of urea to form cyanuric acid is not a unique reaction. Rather, a range of products, in addition to cyanuric acid, can be produced. These products may include amino substituted cyanuric acids, commonly referred to as amides of cyanuric acid, namely ammelide, ammeline, and melamine, as well as other undesirable side products, such as ammonium carbamate and other condensation products.
Furthermore, if careful control is not exercised in producing cyanuric acid by pyrolyzing the urea, numerous side products can be produced. In addition, it is difficult to obtain an easily handled end product in good yield and in a purified form. Where the cyanuric acid is to be chlorinated, it is essential, if satisfactory chlorinated cyanuric acids are to be obtained, that a relatively pure cyanuric acid be used as the starting raw material.
In order to obtain such a purified cyanuric acid, it is the custom in the art to treat crude cyanuric acid to an acid digestion in a strong acid media, e.g., 3 to 15 percent sulfuric or hydrochloric acid. This treatment selectively hydrolyzes the acid soluble ammelide and ammeline and converts them to cyanuric acid. In general, an acid digestion step is required where the concentration of ammeline or ammelide exceeds about 2 percent by weight of the cyanuric acid product.